The electrophysiologic effects of intravenous verapamil were studied in 5 patients with hypertrophic cardiomyopathy. Significant changes occurred in sinus cycle length, atrio-His interval, the pacing rate at which AV nodal block occurred, and the effective refractory period of the AV node. Verapamil appears to act selectively as a potent blocker of AV nodal conduction and should be clinically useful in the control of tachycardia involving this structure.